Consolidate timer_elapsed implementations (#24830)

platforms/timer.c

@@ -6,3 +6,11 @@
 // Generate out-of-line copies for inline functions defined in timer.h.
 extern inline fast_timer_t timer_read_fast(void);
 extern inline fast_timer_t timer_elapsed_fast(fast_timer_t last);
+
+uint16_t timer_elapsed(uint16_t last) {
+    return TIMER_DIFF_16(timer_read(), last);
+}
+
+uint32_t timer_elapsed32(uint32_t last) {
+    return TIMER_DIFF_32(timer_read32(), last);
+}